1. Field of the Invention
The present invention relates generally to cardioversion therapy as delivered by implantable cardioverter defibrillator (ICD) systems for treating ventricular tachycardias. More particularly, the present invention relates to a stepped cardioversion system that decreases the possibility of inducing fibrillation during delivery of cardioversion therapy by the ICD system.
2. Background of the Invention
The use of implantable cardioverter defibrillator (ICD) systems as a medical therapy for persons with abnormal heart conditions or arrhythmias is well known. Initially, ICD systems were used only to recessitate or defibrillate a heart which had stopped pumping because there was no organized heart beat. This type of arrhythmia, referred to as ventricular fibrillation (VF), is relatively simple to detect and is fatal if not corrected in a few minutes. The general approach in using an ICD systems to treat ventricular fibrillation is to deliver a relatively large electrical defibrillation countershock to electrodes implanted about the heart in an attempt to restart the electrical activity of the heart. In existing ICD systems, the defibrillation electrical countershocks are in the range of 25 to 40 joules, and are generated by high voltage capacitors within the ICD system that are charged to approximately 600 to 750 volts by an internal battery.
ICD systems are now being used to treat other types of abnormal heart conditions, such as the main pumping chambers of the heart beating too fast. This type of arrhythmia, referred to as ventricular tachycardia (VT) can be clinically divided into two subclasses. The first VT subclass is a low rate ventricular tachycardia where the heart is beating in the range of approximately 120 to about 180 beats per minute. While a low rate VT is not normal, the patient is not in immediate danger of dying because there is still a perfusing pulse that can pump blood to the body. The second VT subclass is a high rate ventricular tachycardia where the heart is beating in the range of approximately 180 to about 250 beat per minute. In contrast to low rate VT, a patient with a high rate VT is in imminent danger of death within the next several minutes due to a significantly diminished or absent perfusing pulse.
High rate VT, despite its severity and grim prognosis, is treated differently from ventricular fibrillation. This is because, unlike a VF arrhythmia where there is no organized electrical activity of the heart, a high rate VT arrhythmia still exhibits a fairly organized and synchronous electrical activity of the heart and often can be treated by delivering a synchronized "cardioversion" countershock of lower energy that is in the range of 1 to 5 joules. If this cardioversion countershock is unsuccessful, existing ICD systems immediately resort to the use of a defibrillation countershock due to the serious nature of the high rate VT arrhythmia.
Low rate VT is also characterized by a synchronized electrical activity of the heart, but a low rate VT is usually able to generate a perfusing pulse. As a result, it is important in treating a low rate VT to avoid subjecting the patient to an electrical cardioversion therapy that could convert the patient from an abnormal but life sustaining arrhythmia to an abnormal and terminal arrhythmia. Because a low rate VT can be inadvertently converted is not immediately life-threatening, avoidance of shock pain is a major goal. Thus, the usual approach for low rate VT is to deliver bursts of overdrive pacing pulses that will pace the heart at a rate greater than the low rate tachycardia. This technique utilizes pacemaker level energies of approximately 10 to 50 microjoules per pulse for a burst duration of approximately 10 pulses per burst. If the first burst is unsuccessful and the patient remains in a low rate VT, subsequent bursts are reattempted. Unfortunately, even this "Anti-tachycardia" pacing can cause acceleration of the rhythm into a high rate tachycardia or even fibrillation.
In summary, while existing ICD systems have been used with some success to treat low rate and high rate ventricular tachycardias, there is a chance that the existing treatment therapies used by ICD systems for these types of arrhythmias can cause the arrhythmia to further deteriorate to a fatal ventricular fibrillation condition. Consequently, it would be desirable to provide a new treatment therapy for treating ventricular tachycardia arrhythmias using an ICD system that decreases the possibility of inducing fibrillation during delivery of the cardioversion therapy by the ICD system.